1. Field of the Invention:
The present invention relates to an adaptive type signal estimator and, more particularly, to a signal estimator for reducing estimation errors in estimating a transmission signal sequence from a reception signal influenced by noise, fading that changes over time, and the like in an environment in which multipath distortion exists as in mobile communication. A reception signal to be processed is, for example, a signal accompanied by nonlinear distortion such as in narrow-band GMSK (Gaussian filtered Minimum Shift Keying).
2. Description of the Prior Art:
As a conventional device for estimating a transmission signal sequence from a nonlinearly distorted reception signal, an automatic equalizer using a RAM (Random Access Memory) is available.
FIG. 1 is a block diagram showing such an automatic equalizer, which is disclosed in Japanese Unexamined Patent Publication No. 5-14126 (xe2x80x9cautomatic equalizerxe2x80x9d). Binary data of (+1, xe2x88x921) is transmitted from the transmitting side upon GMSK modulation, and the signal influenced by multipath distortion in a transmission path is received on the receiving side. In general, GMSK modulation is a nonlinear modulation scheme which limits the signal band through a Gaussian filter. For this reason, this scheme produces intersymbol interference containing nonlinear distortion in a modulated signal. As the frequency band of a modulated signal is narrowed, in particular, nonlinear distortion increases. In mobile communication, a signal is influenced by multipath distortion in a transmission path. This multipath distortion leads to intersymbol interference due to linear distortion. On the receiving side, therefore, a signal containing both linear distortion and nonlinear distortion is received. In this case, as a distortion model, intersymbol interference spread across five symbol intervals is used. That is, a reception signal xn is influenced by five transmission data sequences (un+2, un+1, un, unxe2x88x921, unxe2x88x922). This phenomenon can be mathematically expressed as xn=f[un+2, un+1, un, uxe2x88x921, unxe2x88x922]. A function f is a function containing both linear distortion and nonlinear distortion.
Referring to FIG. 1, an estimation signal storage unit 204 generates all signals that can be received in a noiseless condition as sample signals. That is, 32 types of sample signals represented by y31=g[+1, +1, +1, +1, +1] are generated from y0=g[xe2x88x921, xe2x88x921, xe2x88x921, xe2x88x921, xe2x88x921]. These signals are stored in a programmable memory typified by a RAM. The function g is a function that is obtained by estimating the function f on the receiving side.
An error detector 201 obtains 32 types of branch metrics required for arithmetic operation based on a Viterbi algorithm from the 32 types of sample signals (y0 to y31) and the reception signal (xn).
A comparator 202 determines a signal by performing arithmetic operation based on the Viterbi algorithm using the 32 types of branch metrics output from the error detector 201.
In general mobile communication, since users communicate with each other on move, multipath distortion also changes over time. That is, the function f changes with time. Therefore, the sample signals stored in the estimation signal storage unit 204 must be changed in accordance with changes in multipath distortion over time. The sample signals are corrected as follows.
To correct the sample signals stored in the estimation signal storage unit 204, a corrector 205 and address generator 203 are required. The address generator 203 sequentially stores the data determined by the comparator 202, and generates an estimated value (wn+2, wn+1, wn, wnxe2x88x921, wnxe2x88x922) of transmission data un. In accordance with the output from the address generator 203, the estimation signal storage unit 204 outputs yn=g[wn+2, wn+1, wn, wnxe2x88x921, wnxe2x88x922] as a sample signal. The corrector 205 then corrects the sample signal yn so it approaches the reception signal xn. By performing this operation sequentially, the 32 types of sample signals stored in the estimation signal storage unit 204 gradually change. This reduces to the function g always tracking changes in the function f.
As described above, since the distortion contained in a reception signal changes with time, sample signals must always be corrected in accordance with the changes in distortion. As a method for this correction, a method of correcting a sample signal by comparing the sample signal obtained from a determination signal with a reception signal is available. In this method, since only one sample signal can be corrected by using one reception signal, an average of 32 reception signals must be input to correct all the 32 types of sample signals.
A scheme that cannot cope with nonlinear distortion, e.g., a scheme using a transversal equalizer, can correct all sample signals by using one reception signal, and hence can track changes in distortion over time no matter how the changes increase. In this scheme, however, since the tracking speed decreases to 1/32, the tracking performance greatly deteriorates.
In addition, as intersymbol interference is prolonged, the number of sample signals exponentially increases. For this reason, the hardware size increases, and the tracking speed decreases exponentially as well.
The present invention has been made in consideration of the above situation in the prior art, and has as its object to provide an adaptive type signal estimator which receives a signal accompanied by nonlinear distortion, and can quickly estimate a transmission signal sequence even if the reception signal is influenced by noise, fading that changes over time, and the like.
In order to achieve the above object, according to the main aspect of the present invention, there is provided an adaptive type signal estimator comprising an estimation signal storage unit for outputting a first sample signal to which nonlinear distortion is added, a convolution arithmetic unit for adding linear distortion to the first sample signal and outputting the signal as a second sample signal, a branch metric arithmetic unit for generating a branch metric used for Viterbi algorithm operation from a reception signal and the second sample signal, and outputting the branch metric, a signal determination unit for outputting a determination signal by performing maximum-likelihood sequence estimation using an output from the branch metric arithmetic unit on the basis of a Viterbi algorithm, and a coefficient corrector for receiving the first sample signal, the reception signal, and the determination signal, and correcting the second sample signal output from the convolution arithmetic unit, wherein the coefficient corrector obtains a difference signal on the basis of a replica generated from the reception signal and the first sample signal and a delayed reception signal obtained by delaying the reception signal by a predetermined period of time, obtains a product signal by multiplying the difference signal by a convergence factor, updates an impulse response value by using the product signal, and outputs the updated impulse response value to the convolution arithmetic unit.
According to the present invention, in obtaining sample signals used to generate branch metrics in a Viterbi algorithm, nonlinear distortion caused in a modulator on the transmitting side and linear distortion caused by a multipath effect in a transmission path are taken into consideration separately to perform adaptive control on only linear distortion that changes over time so as to track only changes in linear distortion over time, but no adaptive control is performed on nonlinear distortion that does not change over time.
To obtain a sample signal, a signal that does not change over time is generated by using a memory table in consideration of nonlinear distortion, and linear distortion is added to the signal. Sample signals can be made to always track changes in linear distortion over time by always correcting linear distortion components used in convolutional operation on the basis of a reception signal and a determination result about the reception signal.
According to the present invention described above, a reception signal can be estimated in an environment including both nonlinear distortion typified by distortion in nonlinear modulation and linear distortion typified by multipath distortion that changes over time. In the prior art, in particular, the response speed of adaptive control to track changes in multipath distortion over time is low, and a large memory capacity is required. In contrast to this, according to the present invention, these problems can be solved by performing estimation processing separately for nonlinear distortion and linear distortion. In mobile communication using narrow-band GMSK, which is a modulation scheme effectively using a frequency band and attaining high power efficiency in transmitters, an improvement in performance and a reduction in apparatus size can be achieved by the present invention in communication using a transmission path in which multipath distortion changes over time. The effect of the present invention is great especially in a field in which the size of a device such as a portable terminal and power consumption are important factors.